Sunday, February 27, 2011

field hiding by subclass, learning notes

(c# has the trickiest hiding rules; c++ has  more than java.)

First, remember the onion model described in other posts. To rephrase one more time, each instance field occupies memory "cells" in a layer of the onion. If class B has an instance field vf, and subclass C has a same-name instance field vf, then a C "onion" has storage for both fields.

myRef.vf resolves to one of the 2 fields at compile time depending on declared type of myRef. P40 [[java precisely]]

On P25 [[java precisely]], B has a static field sf, and so does C. The C onion in memory has NO storage for either field, but it has a pointer to the class object, which has storage for the field.

Q: does the C class object have storage for both static fields?
%%A: probably no. C class object just needs a pointer to B class object.

Question: "myRef.sf" refers to ...? P40 [[java precisely]]
A: At compile time, if the C onion is declared as type B, then the field is B.sf
A: At compile time, if the C onion is declared as type C, then the field is C.sf

I feel the static and non-static cases are similar -- both compile-time binding. This is the simple rule for field binding. For method call binding, static and non-static behave differently. Remember that runtime binding is very strict and requires non-static and non-field, among other conditions. Failing any condition means compile-time binding.

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